I have to confess that I never have used anything like these devices. I have always used the tool-against-ruler trick, which of course is a bit subjective on whether the ruler is absolutely vertical, but it has worked more than well enough. For a facing tool, of course, the ultimate test is to face off and see if it leaves a tit. (With a QCTP, it is so quick to change the height that this is the easiest way to get it dead center - but maybe not such a great way to do it if you have to shim!)